Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream electronically so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is particularly well suited for application of characters onto irregularly shaped surfaces, including, for example, the bottom of beverage containers.
In general, an ink jet composition must meet certain rigid requirements to be useful in ink jet printing operations. These relate to viscosity, resistivity, solubility, compatibility of components and wetability of the substrate. Further, the ink must be quick-drying and smear resistant, must be capable of passing through the ink jet nozzle without clogging, and must permit rapid cleanup of the machine components with minimum effort.
Many of the surfaces on which ink jet printing is utilized are nonporous, such as the above-mentioned beverage container. The ink used for these applications must adhere well to the nonporous substrate and normally must be water-resistant. Presently used ink for such applications conventionally contains a resin, a dye, a carrier, and other components.
Heretofore, to obtain satisfactory adhesion of a water-resistant ink jet composition to a nonporous substrate, it has been necessary to use an organic solvent as the carrier to dissolve the resin and/or the dye. For example, U.S. Pat. No. 4,260,531 discloses an ink jet printing composition comprising a styrene-acrylic copolymer resin, a dye, and a mixture of water and one or more water-miscible C.sub.1 -C.sub.3 alcohols as the diluent. U.S. Pat. No. 4,246,154 discloses an ink jet printing composition comprising an aqueous vinyl polymer suspension which is added to a solution of a hydrophobic dye previously dissolved in an organic solvent.
Use of these organic solvents poses toxicity and flammability problems for the ink jet formulator. However, the use of organic solvents is undesirable for an additional reason--they pose an environmental hazard. After the ink has been applied to the substrate, the solvent evaporates, releasing organic compounds into the atmosphere. These volatile organic compounds adversely affect the environment in the atmosphere and are the subject of both federal and state governmental regulations.
Volatile organic compounds are defined as any compound of carbon that evaporates from a paint or coating film under specific test conditions. The amount of volatile organic compounds, present in a given composition in grams per liter of coating, also known as volatile organic content (VOC), can be determined by using the following formula: ##EQU1## where W.sub.s is the weight of volatile compounds in grams;
W.sub.w is the weight of water in grams; PA1 W.sub.es is the weight of exempt compounds in grams; PA1 V.sub.m is the volume of material in liters; PA1 V.sub.w is the volume of water in liters; and PA1 V.sub.es is the volume of exempt compounds in liters.
In the above formula, exempt compounds are volatile organic compounds whose use is specifically sanctioned by regulatory agencies. Trichloroethane is an example of an exempt compound.
The maximum acceptable level of volatile organic compounds varies for different coating applications. For example, the maximum acceptable level of volatile organic compounds set for the aerospace coatings industry is 340 grams/liter. However, until the present invention, there was no ink jet composition that met any of the government standards and still had the qualities necessary for satisfactory use in ink jet printing operations on nonporous substrates.
From the foregoing it is clear that a general need exists for a substantially organic solvent-free, ink jet composition that adheres to nonporous substrates and is water-resistant, as well as for an ink jet composition which poses minimal toxicity and flammability problems, while being easily disposable by the formulator and end user. A specific need also exists for an ink jet composition suitable for use in ink jet printing operations on nonporous substrates which has a volatile organic compound level less than 500 grams/liter, preferably less than 340 grams/liter. Although such needs in the art have existed for years, no practical solution has yet been found.
Certain specialized application needs also exist which remain unfilled by present-day ink jet formulations. In many instances printed images formed from the ink jet printing operation are printed onto substrates which must be sterilized, as through steam sterilization. For such applications, the ink jet formulation must be capable of forming printed images that remain substantially unaffected after retorting. Such retortability is especially critical with respect to containers for use in the food industry. Generally, such containers are made of glass or are cans made of tin-free steel ("TFS") or aluminum.
In other instances, the substrate with the printed image thereon must be exposed to aqueous and/or nonaqueous solvent-washing procedures. For such applications the jet ink must be capable of forming printed images that withstand solvents, such as isopropanol, ethanol, water, and mixtures thereof, and in other instances, contact with methylethylketone ("MEK") or methanol.
Heretofore, ink jet printing formulations meeting the aforementioned and other needs have not been available, or have been available only in formulations that contain large quantities of volatile solvents. Such needs are now satisfied by the present invention, the description of which follows.